1. Field of the Invention
The present invention relates to a method and apparatus for detecting and processing signals. More particularly, the present invention relates to a signal detector having improved sensitivity and output signal-to-noise ratio.
2. Discussion of the Related Art
FIG. 13 shows an example of a square law detector. Square law detectors typically include a band pass filter 100, processing circuit 200, and a low pass filter 300. In simplified form, processing circuit 200 may include a power divider 210 and a mixer 220.
In a general sense, square law detectors are used to detect the presence of a received signal. Thresholding circuitry, which is typically connected to the output of the square law detector, is then used to detect a change in state, such as a pulse, in the signal received from the square law detector. The ability of a detector to provide a signal to the thresholding circuitry which the thresholding circuitry can distinguish one state from another, is limited by the detector's ability to detect the presence of the signal when ambient noise is also present. Moreover, it is only in rare circumstances that ambient noise is not present. Thus, performance of a detector is typically described in terms of its sensitivity, i.e., its ability to detect the presence of a signal when the level of the received noise is relatively high with respect to the level of the received signal. A measure of the relative levels of a signal and the ambient noise is expressed as a signal-to-noise ratio (SNR). The higher the SNR of a received signal, the more likely a detector will be able to detect the presence of a signal which the threshold circuitry can detect a change in the signal's state. For each detector there exists a minimum SNR of a received signal at which detector can effectively detect the presence of a signal. Hence, the greater a detector's sensitivity, the lower the SNR which the received signal is required to have in order for the thresholding circuitry to accurately detect a change in state.
Although ambient noise may exist at all frequencies, a square law detector can be used to increase the output SNR of a received signal by selecting bandwidths for the band pass and low pass filters which allow the received signal to be passed therethrough, but which prevent some of the noise from passing through. However, the filters cannot prevent the noise which is present within the frequency bandwidth of the received signal from passing through. Therefore, square law detectors are limited in their capability for enhancing the SNR of a received input signal--particularly, in an Electronic Countermeasures (ECM) application in which the bandwidth of the filters must be large in order to allow detection of a large number of possible signals. If the SNR of the received input signal is insufficient, a square law detector will not provide the output SNR that is required for the threshold circuitry to detect a change in the signal state. Thus, the sensitivity of a square law detector is also limited in this respect.